1. Field of the Invention
This invention relates to a scattered light type smoke detector which is capable of enhancing reduction of noises due to the internal reflection thereof.
2. Description of Prior Art
There has been proposed a scattered light type smoke detector adapted with a view to lowering a noise level in which a light source and a photo-cell are so disposed that the optical axes thereof intersect each other at a specific angle so as to prevent the light from the light source from directly entering the photo-cell and a shading member is provided in the vicinity of a smoke detecting area where the optical axes intersect each other to prevent the light from the light source from passing around to the photo-cell and to prevent possible misoperation due to condensation caused on the inside of the detector and insects etc. attached thereto.
A more specific structure for reducing a noise level has been further proposed wherein a conical portion is formed on the bottom of a smoke detecting holder at a position under the smoke detecting area so as to reflect the incident light from the light source deflected from the smoke detecting area or reflected light into the direction other than the direction to the photo-cell so as not to enter the light into the photo-cell for reduction of the noise level due to the inner reflection.
In this connection, it is to be noted that, in the scattered light type smoke detector, weak scattered light irregularly reflected by fine particles of smoke is detected by a photo-cell and therefore noise light due to the internal reflection of the light from the light source should be minimized. To this end, various means are proposed in the above-mentioned prior arts for lowering the noise light, but there have been no smoke detectors which successfully improve the inner surface of the smoke holder to reduce the noise light. By this reason, there remains such a problem that the reflected light within the smoke holder is reflected from said inner surface of the holder to enter the photo-cell to raise a noise level.
There is another problem that since the tip end of the shading board is positioned in the vicinity of the smoke detecting area, water drop attached to the tip end of the shading board causes the light to be bent or reflected to enter the photo-cell, causing misoperation.
Although the light towards the surface of the conical portion can be reflected into the direction so as not to enter the photo-cell, there is a further problem that the light incident on the apex of the conical portion is reflected by the apex into directions of 180.degree. and the reflected light possibly enters the photo-cell. Thus, there is a limit in the prior arts in reduction of noise level due to the internal reflection.
The conventional scattered light type smoke detectors which are proposed to reduce the noise level include smoke detectors taught by U.S. Pat. No. 4,099,065 issued July 4, 1978 to William J. Malinowski, U.S. Pat. No. 4,112,310 issued Sept. 5, 1978 to William J. Malinowski, U.S. Pat. No. 4,216,377 issued Aug. 5, 1980 to Mitsuo Hasegawa, Yoshinori Shinohara, Takashi Suzuki and Akira Yokota. However, none of these detectors can solve the problems as mentioned above satisfactorily.
In addition, it is required for the scattered light type smoke detector to obtain a certain output voltage of the photo-cell for providing a required sensitivity to the incoming smoke having a cetain extinction modulus. For example according to UL Standards which is known as the most strict standards, it is required to obtain a predetermined fire detection sensitivity at an extinction modulus of 4%/ft or less when gray smoke produced upon burning of cotton wick enters and at an extinction modulus of 10%/ft or less upon entering of black smoke produced by burning of kerosine etc.